Diffuse sources include stormwater, atmospheric deposition on water, leaching from forest land, agricultural land and other land areas (mountain land, mire land and open, unforested land) and local on-site waste water treatment facilities. Point sources consist of waste water treatment plants and industries.
Stormwater consists of rain or meltwater that runs off urban areas and road surfaces. Stormwater can be led directly to the recipient, to waste water treatment plants, or it can be treated in some type of stormwater treatment, usually wetland or dam, before it is led to the recipient. Land use and human activity, for example traffic, infrastructure and buildings, affect the composition of stormwater. In addition, atmospheric deposition contributes to the substances that may occur in stormwater.
There are many emission sources to air, such as traffic, combustion and shipping. Airborne substances can travel for long distances and both long distance transports and more local emission sources contribute to the deposited substances. Atmospheric deposition can occur directly on waterbody or indirect through deposition on land and hard surfaces and further through transport with soil water to watercourses, lakes and coastal waters.
Leaching from forested land and other land
The atmospheric contribution is of great importance for the concentration levels of pollutants in the top layer of the soils. The concentration of substances in soils also varies from one place to another depending on local emission sources and variations in deposition, land type and how the substance is transported within a catchment basin. Substances that are stored in the soil, such as nutrients and heavy metals, can be released by activities that affect the topsoil layer (e.g. deforestation, fertilisation, ditching, land preparation and construction of roads) and can lead to an increased leaching to rivers, lakes and coastal waters. Changed chemical conditions of the soil caused by for example acidification also affect leaching and transportation of certain substances.
Leaching from agricultural land
The atmospheric deposition is of great importance for the concentration of pollutants in the top layer of agricultural land. Other sources can be the spreading of sludge (nutrients, metals and organic substances), use of commercial fertilisers (cadmium) and mineral feed supplements that end up in the soil through the use of animal manure (zinc, copper and nickel). Metals and other substances are removed from agricultural land either through harvesting or by leaching to rivers. To what extent substances are leached depends partly on how easily soluble they are and partly to what extent they are adsorbed by crops. Agriculture also stands for a large part of the anthropogenic nutrient inflow to Swedish watercourses.
Local on-site wastewater treatment
Emissions of metals and other substances from local on-site wastewater treatment facilities originate for example from various goods and products used by the household. Locally, where there are many facilities with local on-site wastewater treatment, the load of nutrients to water can be of significance.
Larger point sources – industries and wastewater treatment plants
Emissions of hazardous substances from point sources have a long history due to the industrial development. Industrial activities are both a source of pollutants directly to water as well as indirect, e.g. through long-range transport and deposition of air pollutants.
Wastewater treatment plants belong to the category point sources, however most substances that are emitted by these facilities originate from diffuse sources upstream of the treatment plants such as households, stormwater, infrastructure, industries and a number of smaller local activities.
Further information on industrial emissions and the contribution from wastewater treatment plants can be found on ”Swedish Pollutant Release and Transfer Register”/Search.